One major advantage of the free piston Stirling engine is that the working gas can be entirely sealed within the engine to prevent its contamination or loss by leakage. It is undesirable to lubricate the pistons of the free piston Stirling engine with traditional lubricants such as petroleum based oils and greases because such lubricants vaporize and also contaminate the heat exchangers, reducing their efficiency.
Nevertheless, it is still desirable to lubricate such engines for the purpose of extending the life of the engine components and reducing wear and maintenance.
Current methods of reducing wear and maintenance include hydrostatic gas bearings, in which a gas is pumped through holes in the bearing wall. Solid lubricant bearings are also used in which a material like teflon, for example, coats the surfaces in order to reduce wear and galling. Additionally, some bearing surfaces are formed by providing a very hard surface, typically chromium oxide, alumina, or similar materials which are used to resist wear by virtue of their extreme hardness. Finally, some Stirling engines use conventional oil lubricated bearings which are separated from the working gas by a diaphragm or wiper seal.
It is therefore, an object of the present invention to effect engine lubrication through use of a fluid which acts or is acted upon cyclically and particularly to lubricate the pistons of a Stirling engine with the working gas of the engine.